This invention relates to circuits for compensating for variations in electronic component parameters with time and temperature, particularly to a circuit for reducing the effect of changes in resistances within a multiplying digital-to-analog converter.
CMOS multiplying digital-to-analog converters ("MDACs") are used for many applications requiring an adjustable gain stage. The MDAC ordinarily provides the input and feedback resistors for a conventional linear operational amplifier circuit, the MDAC permitting the input resistance to be digitally selected in response to a binary input signal. The MDAC is made of semiconductor resistance elements and CMOS FET switches in a standard ladder configuration.
The resistance of semiconductor resistors varies greatly with temperature and time. This variation is on the order of 30% of their nominal value. However, since a feedback resistor is included in an MDAC and the feedback resistor is made of the same material as the ladder network, where the MDAC is used in a conventional linear operational amplifier circuit the variation in resistance of the input resistor is cancelled out by the variation in resistance of the feedback resistor, and the gain of the amplifier is dependent only on the settings of the FET switches.
However, there are circuit applications where it is desirable to use an MDAC other than in a linear operational amplifier having a selectable input resistor and a feedback resistor provided by the MDAC. For example, it is often desirable to use an integrator having a selectable input resistor and a capacitor as a feedback element. Since the feedback resistor of an MDAC cannot be employed to provide feedback in such applications, the use of an MDAC in such applications is prone to produce large inaccuracies.
It would be desirable to have a circuit that would compensate for variations in the resistances of an MDAC without requiring that its feedback resistor be used as the feedback element of a linear operational amplifier circuit.